The functions of the spinal column are to provide stability and mobility, protect the spinal cord and control transmittance of the movement of the upper and lower extremities. Spinal stability is commonly defined as the ability of the spine, under physiological loads, to maintain its pattern of displacement so that there is no initial or additional neurological deficit, no major deformity, and no incapacitating pain [1]. Instabilities can arise due to many factors including trauma, degeneracy or metastasis which may result in pain, neurological deficit or even loss of mobility.
Several techniques requiring a wide range of device setups have been developed over the years to restore stability to a compromised spinal column. While the many techniques differ greatly in their implementation, they all serve the same purpose: spinal fusion. Spinal fusion is the process by which two or more vertebral levels are fused together with bone grafts and internal instrumentation to heal into a single, solid bone mass. The process eliminates motion between vertebral segments, which may be necessary to eliminate pain or re-stabilize the spine.
Currently, fusion is accomplished anteriorly, posteriorly or via a synergism of the two. The major anterior approach for fusion is an interbody fusion in which a device having variable height is inserted in the disc space between adjacent vertebral levels to replace part or all of a damaged disc or to restore geometry to a collapsed vertebral body. The interbody device comprises a hollow cylinder in which bone graft is packed to promote fusion of the adjacent levels and osseointegration of the implant. In many cases, the native forces experienced in the spine will require a supplemental posterior fusion or stabilization to re-enforce the anterior instrumentation.
Currently, posterior fusion is predominated by pedicle screw-rod systems. Pedicle screw fixation was first described in North America by Harrington and Tullos in 1969 but did not gain full acceptance until the early part of the 1980s. In transpedicular screw fixation, screws are passed in an anteromedial direction through the pedicles of a vertebra and into the body centrum of the same vertebra. One of two screw trajectories can be used: the anatomical or the straight-forward approach. The anatomical approach, used by the majority of surgeons today, provides the largest possible bone channel for the placement of the screw, but requires the use of poly-axial screws which are locked in place and joined together through rod or plate linkages.
Installation of pedicle screws is heavily dependent on surgical expertise. The angle of insertion into the pedicle is paramount to complication avoidance and even minor misalignments can lead to insult of the vertebral artery in the cervical region if the placement is too lateral. Implications of medial violation of the pedicle can be severe neurological deficit in any region of the vertebral column. The alignment of these screws is a difficult task due to the variability present in the transverse pedicle width throughout the vertebral column. This range of pedicle widths dictates the angle of insertion of the screw.
The expanding knowledge of spinal column biomechanics and the refinements in material selection has slowly shifted the dangers associated with spinal instrumentation from device failures to surgical proficiency. As seen with pedicle screw-rod systems, the dangers encountered are predominantly related to the anatomy of the posterior spine, as described above. Improper insertion of the pedicle screws can lead to insult to the vertebral artery or intrusion into the spinal canal, leading to severe neurological deficit. Due to the risks associated with the procedure, extreme caution is necessary for proper installation. This has led to large surgical exposure for extended periods of time which, in turn, results in increased patient blood loss intraoperatively and longer duration recovery times.
Alternative posterior techniques have been attempted in the past which address some of the inherent risks associated with the pedicle screw-rod systems. In these techniques, adjacent spinous processes are wired together via holes created in the spinous processes. In the case of the Roger's approach the wires are used independently as the method of fixation while the Bohlman's and Dewar procedures incorporate bone graft to supplement the wiring. These techniques have all fallen out of favour owing to their inability to provide sufficient motion restriction for bony fusion formation. Moreover, these techniques are capable of resisting flexion (tension) but not extension (compression) since they rely on wires to hold the vertebrae together. Even with the addition of a bone graft supplement, the insecure fitting of the graft permits levels of motion detrimental to the fusion process.
It is possible to accomplish a posterior spinal fusion through the use of plating systems which contact the vertebrae via the spinous processes. The use of plates allows for motion restriction in both flexion and extension, thus enabling the necessary constraint needed for a healthy bone fusion to occur. In general, these plating systems will comprise of a pair of plates placed on each lateral side of the spine and connected via cross-posts. The plates may be found in various sizes and shapes to accommodate the large diversity of spine morphologies found in the general population.
U.S. Patent Publication No. 2003/0040746 issued to Mitchell, Landry et al. discloses a system which incorporates two plates positioned on contralateral sides of the spinous processes and coupled together with bolts passed through holes, which were pre-drilled in the cortical bone of the superior and inferior spinous processes involved in the fusion. Although this device begins to address the risks involved in the pedicle screw-rod systems and accomplishes both tensile and compressive force restriction in the spinal column, the mode of implementation does not allow fusion of a larger motion segment, nor does it accommodate the natural kyphotic or lordotic curvature over the restricted motion segment. Moreover, the method of connection of complementary plates requires compromise of the structural integrity of the spinous processes, the very element used for the bone-implant interface.
U.S. Pat. No. 5,527,312 issued to Ray, which is incorporated by reference as if fully set forth herein, describes a system incorporating a facet screw anchor and fixation bar for immobilizing two vertebrae relative to each other. A portion of a fixation bar is wrapped around a portion of a superior vertebra pedicle. The fixation bar is secured to a facet screw anchor and the facet screw anchor is positioned through a facet joint of the superior vertebra and into the base of a transverse process of an inferior vertebra. The fixation bar and facet screw immobilize the superior vertebra and the inferior vertebra.